FIELD SCALE REACTIVE TRANSPORT SIMULATION OF THE SUBSURFACE MIGRATION OF HYPERSALINE SOLUTIONS

Since Hanford operations began in 1943, millions of gallons of radioactive waste have been stored in 177 underground storage tanks in areas known as tank farms. It has been estimated that 67 of the single-shelled tanks are either known or suspected leakers. These leaks contain high-level radioactive waste that can react strongly with the sediments beneath the tanks, which may retard or even enhance radionuclide transport. To address the spatial detail of the geology, temporal resolution of the leak events, and multiple interacting components, simulations of the Hanford S-SX Tank Farm are performed with a parallel version of the CRUNCH reactive transport simulator. Because of the high ionic strength of the supernatant liquid leaking from the tanks, the Pitzer ion activity model for describing chemical thermodynamics is implemented. A series of field scale simulations are carried out to reconstruct the vadose migration of the hypersaline solutions. The coupled effects of flow and transport are studied using both the operator split and global implicit reactive transport formulations. Results demonstrate that the accurate simulation of the system is a nontrivial task, and that it is important to consider all of the interacting components of the system, including high ionic strength corrections for non-ideal behavior (Pitzer formulation), density-dependent fluid flow and aqueous-mineral reactions.